1. Field of the Invention:
This invention relates to an apparatus for the control of the number of idling rotations of an internal combustion engine (hereinafter referred to simply as "engine"), and more particularly to an apparatus for the control of the number of idling rotations of the engine, adapted so that when the number of rotations of the engine under closed-loop control falls below the lower limit of the prescribed range of idling rotations, the engine is shifted to the open-loop control mode and the number of engine rotation is quickly increased to within the prescribed range of idling rotation numbers.
2. Description of the Prior Art:
Heretofore, it has been customary during the so-called idling operation of an engine, i.e. when the operation of the engine is continuing while the throttle valve disposed in the intake manifold of the engine is in a substantially closed state, to control the number of idling rotations of the engine by controlling the amount of air taken into the engine with a control valve disposed in a bypass which communicates with the upstream and downstream sides of the throttle valve.
In other words, during the idling operation, the degree of opening of the control valve is controlled in the closedloop mode so as to ensure supply of the inspired air to the engine in the prescribed amount and approximate the number of idling rotations of the engine to the prescribed level.
To be specific, the exciting current fed to a solenoid which proportionately controls the opening area of the control valve is adapted to be fixed in accordance with the solenoid current command Icmd to be obtained by the following formula (1). EQU Icmd=Ifb(n) (1)
In this formula, Ifb(n) represents the term of PID feedback control for effecting proportional (P term), integral (I term), and differential (D term) control based on the difference between the target number of idling rotations and the existing number of engine rotations.
It is well known that in the engine of the electronically controlled fuel injection type, an increase in the amount of the inspired air results in an increase in the amount of the fuel injected and consequently in the amount of the mixture to be formed.
The conventional technique described above has entailed the following problem.
During the idling operation, the coefficients (control gains) of the P term, I term, and D term are fixed at relatively small levels, while the degree of opening of the control valve is subjected to the feedback control based on the PID feedback control term. This is because the stability of the number of idling rotations during the steady idle operation is rather impaired when the control gains are too large.
During the idle operation of the engine, when the output side of the engine is connected to driving wheels (for the engine to assume an in-gear state) while the throttle valve is kept in a substantially completely closed state, a load is applied on the engine and the number of idling rotations decreases abruptly. In response to this decrease, the value of the feedback control term Ifb(n) and consequently the quantity of the solenoid current command Icmd are increased and the control valve is driven in the direction of opening. Since the control gains of the feedback control term are relatively small as described above, the speeds of response are slow. As the result, the conventional apparatus has had the disadvantage that the engine is liable to stall.
This invention has been produced for the solution of the problem mentioned above.